Radio-frequency current transformer



2 5 INVENTORS HOWARD C. LAWRENCE JR mum/rm .A

ourpur is FREQUENCY IN MEG/467C153 R R FREAS, JR ET AL.

RADIO FREQUENCY CURRENT TRANSFORMER Flled Feb 2, 1946 Oct. 17, 1950 Patented oot. 17, 1950 RADIO-FREQUENCY CURRENT TRANSFORMER Robert Russell Freas, Jr., Jermyn, Pa., and Howard C. Lawrence, Jr., Haddonfield, N. J assignors to Radio Corporation of America, a corporation of Delaware Application February 2, 1946, Serial No. 645,206

1 Claim.

This invention relates to a new and useful radio frequency current transformer.

An object of this invention is to provide a simple and improved current transformer for measuring radio frequency currents. The transformer of this invention has a constant input currentoutput current ratio over very wide frequency limits.

Another object of this invention is to provide a novel current transformer whose secondary will Work into an impedance load in the vicinity of 100 ohms and at the same time provide constant load current for constant primary current over a wide frequency range, such as, for example, two megacycles to twent megacycles per second.

Radio frequency current transformers are often used as isolation devices, and to step-up or stepdown currents, or produce voltages proportional to currents for metering or other purposes.

As is well known to those skilled in the art, a transformer is characterized by two coil windings, called a primary and a secondary respectively, having self and mutual inductance. Current transformers of the prior art only operate satisfactorily at a single frequency, or over narrow band of frequencies, or when the secondary winding is connected to a resistive load low compared to the secondary winding reactance (impedance of the order of 1 ohm over the range 2 to 25 mc., for example).

There are many applications in which it is necessary to provide a current transformer to operate into impedances of the order of 100 ohms over this same range and still operate in such a way as to give a constant primary-current-tosecondary-current ratio over this wide frequency band. The higher impedance connected to one winding (usually the secondary) often requires an increased number of turns in that winding of the current transformer. This increased number of turns increases the radio frequency reactance of the winding and as a result introduces a frequency effect into the resultant output voltage or current. This frequency effect is usually such that the secondary voltage (or current) of the transformer increases as the frequency increases. This increasing voltage or current can be accounted for by the well-known mathematical relationship E=Ldi/dt, wherein E=voltage, L=inductance and the increasing frequency gives an increasing rate of change of current with re- 2 spect to time=(di/dt) Prior art radio frequency transformers are often connected to thermocouples. A thermocouple has a very low impedance. Since the impedance is low, it is not necessary to provide a high amount of secondary voltage to drive the required current through the thermocouple to cause an indication. This allows a low reactance in the transformer secondar winding and it is therefore possible to operate at high frequencies with relatively slight frequency effect. Even thermocouples would not perform as satisfactory loads for current transformers at high frequencies as represented by 25 me. or higher. The exact point at which prior art transformers become unsatisfactory is determined by the individual load impedance and frequency range involved in each application.

Current transformers of the prior art generally were constructed of solid wire and often had higher ouput at higher radio frequencies because of the fact that the rate of change of current per unit of time is greater at these frequencies. In order to cancel out this effect, it is necessary to add an impedance to the secondary which will increase as the radio frequency increases.

If it were possible to insert in the secondary circuit a resistance that would increase with frequency at the same rate as the secondary voltage increases with frequency, the voltage applied to the load, and the current produced b this voltage, could then be maintainedconstant irrespective of frequency. This invention sets forth a ncvel method of automatically inserting this resistance. This automatic method is to construct the high impedance winding of wire comprised of insulated strands, commonly known as Litz wire. Litz wire has a. characteristic that above a certain critical frequency, the radio frequency resistance of the wire increases as the frequency increases. By winding the secondary with this wire, the increased secondary resistance can be automatically obtained. The shape of the resistance-frequency curve is determined by the number and size of the strands in the wire. The number of strands and the diameter of each individual strand is so chosen that the proper resistanceto-frequency relationship is produced.

The transformer of this invention is very useful in circuits where the load resistance is not low compared with the secondary impedance.

According to our invention, we have found that the losses introduced by stranded insulated wire at these high frequencies introduce the desired effect to improve the prior art device, this producing the substantially uniform frequency response. Our transformer invention can be so constructed that resonance with stray capacity of the secondary circuit occurs at a frequency well within the operating range of the current transformer, thus increasing the sensitivity of the device, -by a careful selection of a proper number and size of stranded insulated wires to give the proper inductance to the secondary winding. The resonance curve of the transformer of this invention is found to be very broad because the secondary winding works into a low impedance (approximately 80 ohms). The resultant improved transformer of our invention has both high output and a flat frequency response. Electrostatic coupling between primary and secondary windings is eliminated by a Faraday screen of electrostatic shield placed between the two windings.

This invention will be clearly understood by referring to the accompanying drawing of one reduction to practice of this invention, in which:

Fig. 1 is a top view of the transformer of this invention,

Fig. 2 is a longitudinal cross section of Fig. 1,

Fig. 3 is a simplified circuit diagram showing the use of the transformer of this invention in an antenna circuit of a radio transmitter, and

Fig. 4 is a curve showing the frequency response from the transformer of this invention.

Description and figures given in this application apply to a typical representative transformer and metering circuit.

Referring now in detail to Figs. 1 and 2 of the drawing, a tubular insulating casing member I which is preferably composed of a high grade ceramic material such as, for example, isolantite has located therein a tubular insulating coil support member 2. The support member 2 is preferably made integral with an insulating end member 3. The lower end of casing l is provided with a metal base member 4 which is preferably made of brass and silver plated to provide a low resistance path for the radio frequency currents. Within the inner wall of the ceramic casing I, there is arranged an electrostatic Faraday shield 5 which is preferably composed of a plurality of spaced wires 6 which run parallel with the axis of tube I, the stranded wires 5 being woven with an insulating medium such as a linen thread I. The thread I is woven at right angles to the parallel wires 6. The lower end of wires 6 are all brought out between the lower end of easing l and the inner surface of member 4 and is soldered to the latter at a position indicated at 8. The coil support member 2 has located in its central portion a secondary winding 9 which is composed of twenty-five turns of ten strands of Number 42 B. S. gage copper wire for this particular application. Each of the single strands is coated with enamel to provide the proper insulation. The primary winding I0 is located within a groove IDA on the outer surface of tube I and consists of a single turn of copper bus wire .128 in diameter, the outer surface of the wire being tin plated to prevent oxidation. If more than one turn is desired to give a change in the transformer ratio, the groove IDA may be out similar to a helical thread the pitch of which is determined by the desired spacing of the wires. A movable powdered iron core H is located within an aperture 12 of tube 2 and is arranged to vary coupling of primary [0 with respect to the secondary winding 9, the variation in the position of the core being accomplished by having a threaded flange member II which is secured to member 3 by being knurled at I4 and driven into the upper portion of aperture l2. A threaded rod it extends within the aperture I2 and is secured at one end to the movable iron core II. The upper end of rod I! is slotted at It for rotation with an ordinary screw driver. Member 13 is provided with a slot I! which extends inward slightly beyond the threaded aperture in member H. The rod II is retained in position by a spring wire frictional member ii. The coil support member 2 is threaded at the lower portion thereof to receive a threaded RH screw member is. A spring washer 2a is interposed between the base member 4 and the end of coil support 2. The screw member I! is tightened to bind members I, 2, 4 and I in their proper position. One end of the secondary winding is soldered to a terminal lug 2| and is secured in electrical contact with the plate 4 by means of the spring washer 20 and the screw binding member IS. The other end of secondary coil 9 passes through an aperture 22 in member 3 and is provided with a suitable terminal connection member 23.

Referring now in detail to the circuit diagram, Fig. 3, which shows a typical use of the transformer of this invention, the primary i0 is shown as connected between the transmitter output 33 and an antenna 3|. The plate 4 connects one end of the shield wires 8 to one end of the secondary winding 9. Member 4 is maintained at ground potential as indicated at 32 and connects to one terminal of a milli-ammeter 33. The other terminal of milli-ammeter 33 is connected to a vacuum-tube rectifier device 34, the rectifier being then connected to terminal 23. Noninductive carbon resistor 31 is connected between 23 and 32. Its value is about ohms.

Fig. 4 shows the performance comparison of a transformer of the prior art which generally has its secondary constructed of solid wire 35 and the transformer of this invention which employs stranded wire 36. It will be noted that as the frequency is increased, the voltage across the solid wire secondary increases, whereas with the stranded wire employed by the transformer of this invention, the voltage is substantially constant as the frequency changes, thus giving a substantially uniform frequency response.

The relationship between frequency and the secondary voltage across a load resistor of the order of 100 ohms or a constant primary current of a prior art transformer and a transformer improved by this invention are as shown in Fig. 4. It will be noted that the secondary voltage increases with frequency for the prior art transformer while it is essentially flat for the transformer of the present invention.

The particular transformer of this invention is particularly useful for measuring radio frequency currents covering ranges from two to twenty megacycles per second. The transformer core H may be dispensed with if close coupling between primary and secondary for maximum secondary voltage is not required. Core II also serves as adjustment of secondary voltage for a fixed primary current.

What is claimed is:

A radio frequency current transformer comprising an insulating tubular casing, a metallic member closing one end of said casing, an insulating member closing the other end of said casing, said insulating member having an in- 5 sulating tube portion extending within said insulating casing and secured to said metallic end member, a primary winding located on the outside wall of said insulating tubular casing, a secondary winding located on said insulating member within said insulating tubular casing, an electrostatic shield of parallel spaced wires interposed between said primary and secondary winding, said electrostatic shield and one end of said secondary winding being electrically connected to the said metallic end member, said primary comprising a single turn solid strand copper conductor having a diameter of 128 mils and said secondary being comprised of 25 turns of ten insulated strands of Litzendraht wire of Number 42 B. S. gage copper wire, the number and size of said insulated strands being chosen so as to cause the resistance of said secondary to increase with increase of frequency at a predetermined rate whereby a constant secondary current for a constant primary current is obtained over a frequency band of from 2 to 20 megacycles.

ROBERT RUSSELL FREAS. Jn. HOWARD C. LAWRENCE, Jr.

6 nsmnnces crrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS Num er Name Date 1,320,980 Bowman Nov. 4, 1919 1,709,826 Austin Apr. 23, 1929 1,722,444 Reiche July 30, 1929 1,792,730 Chryst et al Feb. 17, 1931 1,802,371 Builivant et al Apr. 28, 1931 1,829,740 Drake et a1 Nov. 3, 1931 1,997,453 Crossley et al. Apr. 9, 1935 2,058,037 Rigandi Oct. 20, 1936 2,221,217 Kirk et al Nov. 12, 1940 2,225,967 Berman Dec. 24, 1940 2,263,613 Conron Nov. 25, 1941 2,297,476 Holsten Sept. 29, 1942 2,318,271 Weiche May 4, 19443 2,331,101 Beard Oct. 5, 1943 2,374,018 Johnson Apr. 17, 1945 2,375,309 McCoy May 3, 1945 OTHER. REFERENCES Lafayette Radio Catalog No. 78, 1940, page 155. 

